One of the biggest mistakes in projection tools is assuming a return that is too optimistic and then treating the result as a plan instead of a scenario.
A better approach is to model three cases: conservative, base case, and optimistic. That helps you see whether your goal still works when assumptions become less favorable.
A range also keeps you from building confidence around a forecast that markets are under no obligation to deliver.
A stock-heavy portfolio, a mixed stock-bond portfolio, and a mostly-cash portfolio should not all use the same expected return.
The more aggressive the assumed return, the more important it becomes to understand the volatility and uncertainty attached to it.
Nominal balances can look impressive while still buying less in the future. If you care about future purchasing power, run a lower-rate scenario that roughly reflects inflation-adjusted results.
That can make long-term goals feel more realistic and keep you from under-saving.
Use the investment calculator to test at least three return assumptions rather than relying on one.